I went out like a light.
→   금새곯아떨어졌어.

[예문]
A: So how did you sleep last night?
       그래 지난 밤엔 잘 잤어?

B: I slept like a baby.
    곤하게 잘 잤어.

A: You did?
     그랬어?

B: Yes. I went out like a light.
    응. 금새 곯아떨어졌어.